<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <script src="../js/vue.js"></script>
</head>

<body>
  <div id="root">
    <h2>今天天气很{{info}}</h2><br>
    <button @click="changeWeather">改变天气</button>
  </div>
  <script type="text/javascript">
    const vm = new Vue({
      el: '#root',
      data: {
        isHot: true,
      },
      computed: {
        info() {
          return this.isHot ? '炎热' : '凉爽';
        }
      },
      methods: {
        changeWeather() {
          this.isHot = !this.isHot;
        }
      },
      watch: {
        //watch中也可以监视计算属性
        // isHot: {
        //   immediate: true,//初始化时让handler调用下
        //   deep: true,//深度监视
        //   //函数名固定，当isHot发生改变的时候发生调用
        //   handler(newValue, oldValue) {
        //     console.log("isHot被修改了.");
        //     console.log(newValue, "---", oldValue);
        //   }
        // }

        //简写形式,简写的形式前提是只有handler函数，没有immediate和deep属性
        // isHot(newValue, oldValue) {
        //   console.log("isHot被修改了.");
        //   console.log(newValue, "---", oldValue);
        // },
      }
    })

    //正常写法
    /* vm.$watch('isHot',{
      immediate:true, //初始化时让handler调用一下
      deep:true,//深度监视
      handler(newValue,oldValue){
        console.log('isHot被修改了',newValue,oldValue)
      }
    }) */

    //简写
    vm.$watch('isHot', function (newValue, oldValue) {
      console.log('isHot被修改了', newValue, oldValue, this)
    })

  </script>
</body>

</html>